1. Technical Field
The invention relates generally to the field of printing using an electrostatic printing mechanism such as is found, for example, in laser printers and more particularly to a pulse width modulator configured for receiving digital data specifying grayscale values of pixels to be printed and, thereafter, converting the digital data to variable width pulses for driving the electrostatic printing mechanism.
2.Description of the Prior Art
Electrostatic printing mechanisms are popular in devices such as laser printers for reasons which include reliability and the ability to print at high resolutions using indelible toners. Digital information which is supplied to the electrostatic printing mechanism typically specifies grayscale intensity of pixels to be printed as an eight bit value corresponding to each pixel. In order to be used by the electrostatic printing mechanism, this digital information must be converted to a format appropriate for directly driving the laser or other such printing mechanism which is being used. The standard format uses a pulse width modulation scheme that converts eight bit pixel intensity values to predetermined pulse widths. In this regard, it should be appreciated that an eight bit value is capable of specifying 256 discrete values which may correspond to 256 different pulse widths, each of which represents a different grayscale level. It should also be appreciated that conversion of the grayscale digital pixel values to varying width pulses commands the need for relatively high data throughput in state-of-the-art systems and, of course, in future systems. For example, for video clock rate of 10 MHz, the conversion process would operate at approximately 2.5 GHz. This latter rate is significantly higher than what is presently available using pure digital technology.
One approach seen in the prior art for accomplishing the conversion process at an acceptable output rate uses analog circuitry to charge a capacitor while measuring the charge time of the capacitor. This approach is exemplified, for example, by a device designated as the AD9561 pulse width modulator manufactured by Analog Devices, Inc. While this device and similar devices that also utilize the approach are generally suited for their intended purpose, several limitations have come to light with regard to their operation. For example, in one limitation, the analog nature of the operation of these devices is relatively sensitive to noise. Another limitation resides in the tendency of these devices to be subject to drifting such that recalibration is required at frequent intervals in order to compensate for this drifting.
The present invention provides a highly advantageous pulse width modulator for use in electrostatic printing applications which eliminates the foregoing limitations by using a hybrid analog/digital approach.